44 lines
2.5 KiB
Markdown
44 lines
2.5 KiB
Markdown
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# Proof of History
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[Proof of History overview](https://medium.com/solana-labs/proof-of-history-a-clock-for-blockchain-cf47a61a9274)
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## Relationship to consensus mechanisms
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Most confusingly, a Proof of History (PoH) is more similar to a Verifiable
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Delay Function (VDF) than a Proof of Work or Proof of Stake consensus
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mechanism. The name unfortunately requires some historical context to
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understand. Proof of History was developed by Anatoly Yakovenko in November of
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2017, roughly 6 months before we saw a [paper using the term
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VDF](https://eprint.iacr.org/2018/601.pdf). At that time, it was commonplace to
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publish new proofs of some desirable property used to build most any blockchain
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component. Some time shortly after, the crypto community began charting out all
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the different consensus mechanisms and because most of them started with "Proof
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of", the prefix became synonymous with a "consensus" suffix. Proof of History
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is not a consensus mechanism, but it is used to improve the performance of
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Solana's Proof of Stake consensus. It is also used to improve the performance
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of the replication and storage protocols. To minimize confusion, Solana may
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rebrand PoH to some flavor of the term VDF.
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## Relationship to VDFs
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A desirable property of a VDF is that verification time is very fast. Solana's
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approach to verifying its delay function is proportional to the time it took to
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create it. Split over a 4000 core GPU, it is sufficiently fast for Solana's
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needs, but if you asked the authors the paper cited above, they might tell you
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(and have) that Solana's approach is algorithmically slow it shouldn't be
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called a VDF. We argue the term VDF should represent the category of verifiable
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delay functions and not just the subset with certain performance
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characteristics. Until that's resolved, Solana will likely continue using the
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term PoH for its application-specific VDF.
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Another difference between PoH and VDFs used only for tracking duration, is
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that PoH's hash chain includes hashes of any data the application observed.
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That data is a double-edged sword. On one side, the data "proves history" -
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that the data most certainly existed before hashes after it. On the side, it
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means the application can manipulate the hash chain by changing *when* the data
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is hashed. The PoH chain therefore does not serve as a good source of
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randomness whereas a VDF without that data could. Solana's leader selection
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algorithm (TODO: add link), for example, is derived only from the VDF *height*
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and not its hash at that height.
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